Method and apparatus for taking up fiber

ABSTRACT

An in-line take-up apparatus and process isolate the tensional forces needed in a fiber for good packaging about a wind-up member from the portion of a fiber having very low tensile strength which occurs immediately after extrusion.

United'States Patent Haley et al. [451 July 18, 1972 METHOD ANDAPPARATUS FOR [56] References Cited TAKING UP FIBER UNITED STATESPATENTS [72] Inventors: David J, Haley, Durha R b rt E, C 2,030,1892/1936 Taylor "18/8 B X i Wilbur prim, Jr both of 2,582,639 1/ 1952Ljungberg. ....18/8 B UX l i n of NC. 3,367,399 2/1968 Easton 164/282[73] Assignee: Monsanto Company, St. Louis, Mo. FOREIGN PATENTS ORAPPLICATIONS [22] Filed: June 9, 1970 1,191,555 4/1965 Germany ..l8/8 B383,564 l/l965 Switzerland ..65/1 1 W 21 App1.No.: 44,821

Primary Examiner-Stanley N. Gilreath Attorney-Vance A. Smith, Russell E.Weinkauf, John D. [52] US. Cl ..242/l8 R, 18/8 B, 18/12 TB, Upham andNeal E. Willis 65/11 W, 164/82, 164/282, 242/147 R, 264/176 F [51] Int.Cl ..B65h 54/00 57 T C [58] Field of Search..... ....242/18 R, 18 G,147; 18/1 FZ,

18/8 B, 8 WB, 12 TB, 12 TF, 12 TM; 164/82, 282; 264/176 F; 65/11 R, 11 WAn in-line take-up apparatus and process isolate the tensional forcesneeded in a fiber for good packaging about a wind-up member from theportion of a fiber having very low tensile strength which occursimmediately after extrusion.

17 Claims, 6 Drawing Figures Patented July 18, 1972 0 0 b DISTANCE FROMORIFICE FIG.

INVENTQ S DAVID J. HALEY ROBERT E. CUNNINGHAM WILBUR J. PR] VOTT, JR. BYIflf! A ORNE Y METHOD AND APPARATUS FOR TAKING UP FIBER CROSS-REFERENCESTO RELATED'APPLICATIONS This application is related to copending andcommonly assigned applications Ser. No. 829,2l6, filed June 2, 1969;ofS. A. Dunn, L. F. Rakestraw, andR'. E. Cunningham; Ser. No. 863,311filedOct. 2, 1969, of W. .l. Privott and R. E. Cun ningham; and Ser. No;870,646 filed Oct; 27, 1969 of W. .l. Privott'and R. E. Cunningham:

FIELD OF THE INVENTION The'present inventionrelates to the take-up ofcontinuous fiber and,-more particularly, tothe in-line take-up ofcontinuous fiber extruded from a low viscosity melt.

BACKGROUND OF THE INVENTION Descriptionof the Prior Art It has long beenknown that liquid streams with low viscosities break up into discreteparticles sometimes called drops shortly after issuance from anextrusion assembly. The high surface tension of the liquid relative toits viscosity causes the inherent cross-sectional non-uniformitiesexisting side-by-side in the stream to become more pronounced untilbreak-up occurs. Many materials of interest such as the metals,ceramics, and other inorganic materials-have low viscosities in theliquid phase. Until recently, it was generally consideredto be almostimpossible to extrude these materials as amolten stream and solidify thestream into a continuous fiber prior to break-up due to the surfacetension effect.

A technique of preventing disruptions in stream continuity due tosurface tension has beendeveloped and is discussed in detail inapplication Ser. No. 829,216. Generally, the molten stream is extrudedinto a gaseous atmosphere hereinafter called the stabilizing atmospherewhich reacts and/or decomposes in the presence of the stream and formsafilm about the periphery of the stream. The film has sufficient strengthto prevent surface tension-induced disruptions from taking place in themolten portion of the stream while solidification is occurring. Thus,this technique provides a simple and quick method of producing fiberwhich before its adventwas accomplished via the slower and generallymore complex drawing or glass sheathing techniques.

One troublesome problem which has proven to be difficult to solve isfiber take-up. Inthe continuous take-up of afiber, it is necessary toapply a certain amount of tension to the fiber. When tension is appliedto the fiber attained via a film stabilizing process, it is transferredto'the film which has a very small tenacity, causing the stream to bedisrupted.

Varied take-up procedures have herebefore been utilized. The simplest iscalled back-winding" which merely allows the fiber to accumulate beforewinding proceeds, Back-winding is not only time-consuming and tedious,but it oftenresults in kinks and tangles in the packaged fiber.

Another technique is described and claimed in application Ser. No.870,646 wherein freely falling continuous fiber, especiall'y a finediameter fiber, can be made to fioat down in a helical configuration byproviding a sufficiently large upward acting aerodynamic drag force onthe fiber. The helical configuration isolates the tensional forceapplied to the lower end of the filament from the fragile liquid portionof the stream.

This technique is not self-regulating, however, and necessitatescontinuous and close scrutiny since an increase in winding take-up speed(or decrease in melt extrusion velocity) results in the disappearance ofthe helical buffer and, ultimately, in stream disruption. Conversely, alarge accumulation of fiber occurs when the take-up velocity is slowerthan the extrusion velocity, resulting in a situation analogous tobackwinding.

Consequently, there is a definite need for a continuous andself-regulating technique and apparatus for in-line take-up of acontinuous fiber where it is desired to isolate the tension required forwinding from fragile portions of the fiber. It is a primary object ofthe present invention to provide for an apparatus and method whichfulfills the stated need, particularly in cooperation with low viscosityspinning'processes and assemblies.

SUMMARY OF THE PRESENT INVENTION To a greatextent, the present inventioninvolves the recognition of certain physical forces present intheinter-action of a moving fiber over curved surfaces and themanipulation thereof to accomplish the desired winding-up of the fiberwithoutconcurrent occurrence of tensile breakage in the upstream fragileportions of the fiber. As stated previously, it is necessary for a fiberto be under a certain amount of tension for it to be wound-up properly.The problem heretofore has been that the upstream fragile portions ofthe fiber, as, for example, the liquid portion in an extruded metalstream, cannot withstand even the minimal amount of tensional forceneeded towindafiber. It may be shown that this minimal tensional forceshouldbe pAu where p, A, and v are the density, crosssectional area,andfiber speed, respectively. This amount of tension merely counteractsthe centrifugal force on the wire. Ordinarily, commercial uses and needsdictate that fiber must be wound under even greater tension for goodpackaging. Thus, in order to wind-up an extruded fiber having upstreamfragile portions, it is necessary to isolate the tensional forces fromthe upstream fragile portions of the fiber.

One embodiment of the present invention accomplishes the desiredisolation by providing a take-up apparatus which comprises a firsttensioning means including a concave slide surface for intercepting afiber moving at about a velocity v at a first point on the surface. Thefiber is constrained to move along the concave surface due tocentrifugal force. The major contribution to the tensional force inducedinto the fiber is due to the frictional interaction between the movingfiber and concave surface; It may be shown that the buildup of tensiondue to friction asymtopically approaches a value of pAv Thereare,however, other factors which contribute to the tension-in the fiber suchas aerodynamic drag, friction effect due to the weight of the fiber, andthe change in height of the concave surface. At apoint along the slide,the combined effects of the factors mentioned above induces a tensionalforce of about pAv into the fiber at which point the fiber forms anunsupported loop above the concave surface. That is, the fiber at thesecond point moves away from the concave surface and forms a loopunsupported by the surface.

After the fiber leaves the concave surface, it is necessary to increasethe tensional force to a value above pAu to facilitate wind-up. This maybe accomplished by positioning a convex surface spaced apart from theconcave surface and above the point at which the fiber forms anunsupported loop. The convex surface is not effective to increasetension unless the tensional force is already above pAv because thecentrifugal force of the fiber acts in adirection away from the convexsurface, thus preventing a frictionally induced tensioning force frombeing generated. The gravitational force, however, acting downward onthe fiber as it moves upward to the convex surface increases the tensionin the fiber to a value slightly above pAv, allowing the convex surfaceto be effective in further increasing the tension in the-fiber.

At constant take-up speeds, the position of the unsupported loop withrespect to the concave surface remains unchanged. When the take-upvelocity increases the point at which the unsupported loop forrns tendstomove along the surface since pAu has increased in value. A variationin take-up speed (and therefore in the tensional forces required) iscompensated for by the proper utilization of the concave surface. Thus,the continued presence of the unsupported loop indicates that thesurface is frictionally inducing a tensional force sufficient to providethe minimum force necessary for wind-up.

BRIEF DESCRIPTION OF THE DRAWINGS The novel features believedcharacteristic of the present invention are set forth in the appendedclaims. The invention with further objects and advantages thereof may bebest understood by reference to the following description taken inconnection with the accompanying drawings in which:

FlG. 1 is a graph depicting filament length as a function of theintercept distance measured from the point of issuance of the moltenstream.

HO. 2 is a schematic of a take-up system in accordance with oneembodiment of the present invention.

FIGS. 3, 4, 5 and 6 illustrate schematically various other embodimentsof the present invention.

DESCRIPTION The following discussion describes the utilization of thepresent invention relative to a film stabilization technique inproducing continuous fibers. After a reading of the invention asdescribed, however, it will be realized that this invention may beemployed wherever is in a weakened condition immediately afterextrusion. Thus, the phrase "upstream fragile portions of a fiber" asused herein is meant to include both the liquid region of fibers beingstabilized in accordance with a film stabilization technique and thoseother fibers which when extruded are not initially strong enough nearthe origin of extrusion to withstand the sum total of the tensionalforces needed to wind a fiber about a rotating member and weight of thefree length of fiber.

In the fabrication of continuous filamentary material via a stabilizingtechnique, it is extremely important to catch, intercept, or otherwisedecelerate the falling stream at an appropriate position along its path.Premature deceleration may result in a non-fibrous mass of materialsince insufficient time has elapsed for the molten stream to solidifyinto filamentary form. The point which demarks the boundary betweendecelerating positions attaining non-fibrous mass as opposed to afibrous mass is called D,,. Tardy deceleration results in the formationof staple since the weight of the stream creates a tensional force whichexceeds the strength of the stabilizing film so that the stream breaksin the liquid portion near the orifice. The point above which the streammust be collected to prevent breaks due to its weight is called D,,.

The net tensional force is the resultant of the forces due to gravityand aerodynamic drag acting upon the stream. More explicitly, it may beshown that the maximum net tensional force F (max) acting on a stream ofa length extending from the orifice to the point D,, is given veryclosely by F,(max)=(pAg-l.l3V)D (I) where p is the density of thestream, gms/cm;

A is the cross-sectional area of the stream, cm";

g is the gravitational constant, cmlsec is the viscosity of the gas,poises;

a V is the extrusion velocity of the stream, cm/sec;

D is the length of the stream from the orifice to D cm. and the verticaldownward direction is taken as positive. Thus, from relationship (1) itis seen that it is necessary to decelerate the stream before thetensional forces reach F (max). In other words, the stream must bedecelerated prior to reaching the point D,,.

Deceleration of the stream between points D,, and D,, does not assurecontinuity, however. Because of the physical characteristic of thestream immediately below D,,, the stream will break into staple upondeceleration. As seen in FIG. 1, the length of the staple increases froman intercept at D,, until the fiber becomes continuous at a point D,, wen D,, lies below D,,. Copending application Ser. No. 863,3l 1 discussesand claims the spinning conditions which may be varied to manipulate thepoints D,,, D,,, and D,, to ensure that the points are properlypositioned along the stream.

Briefly, however, the location of the point D,, depends primarily on thestream velocity and the coefficient of heat transfer away from thestream. The location of the point D,, is a function of the streamdiameter, density, stabilizing film strength, gas density, and gasviscosity. The stream diameter and density are variables generallypreset by the type, size, and quantity of fiber desired. Gas density andviscosity are variables generally utilized in optimizing conditionsalready set. Because the small tensile strength possessed by the liquidportion of the stream is primarily due to the film strength, it followsthat the film strength is the most significant variable in determiningD,,. Introduction of the stabilizing gas as close to the extrusionorigin as the stream flow patterns permit is desirable since thisensures that the film forms early in the stream existence and, alongwith the gas concentration, determines to a large extent the strength ofthe film. A film of sufficient strength allows the stream to attain alength which extends beyond D,, i.e., D,, lies below D,,. Increasing theaerodynamic drag between the fiber and surrounding gaseous atmospherealso causes the distance to D,, to increase. To determine whetherconditions are properly set so that D,, lies beneath D,,, a flat surfacemay be inserted into and moved along the path of the stream. When D,,lies below D,,, continuous fibers are formed as the surface enters theregion intermediate the two points.

The primary problem involved in the in-line take-up of fibrous materialsproduced from low viscosity melts by a stabilization technique is therelatively large tensional forces needed in the fibrous materials fortake-up as opposed to the ordinarily much smaller strength of thestabilizing film which holds the liquid portion of the stream together.It may be shown that the minimum tensional force which must be suppliedto a fiber to allow take-up by a rotating member such as a bobbin is:

F, p141! 2) where p is the density of the fibrous material;

A is the cross-sectional area of the fiber;

v is the take-up speed of the fiber.

The maximum force AF that can be applied to the filament at distance ADabove the point D, is given by AF=(pAgl.l3;.tV)AD (3) where AD mustnecessarily be less than the distance between points D,, and D,,.Unfortunately, the tensile force of equation (2) is ordinarily greaterthan the force that can be applied at maximum AD. Generally, the tensileforce needed for wind-up is several times greater than the tensilestrength of the stabilizing film.

it is now evident from the above discussion that in-line takeup of afiber having fragile upstream portions may be accomplished only if thetensional force created by the rotary motion of the winding element isisolated from the fragile portion of filamentary material. This may beaccomplished by imposing a force in a direction along the axis of thefiber below the fragile upstream portions which force has a magnitudesufficient to maintain the tensional force about equal to that requiredto wind the fiber about a bobbin. This relationship may be shown 1) (4)where F,, is the net tensional force acting upon the fragile portion ofthe fiber and F, is the force imposed along the axis of the fiber.

FIG. 2 illustrates a tensioning device 10 which is utilized in providingthe amount of tensional force needed for take-up to fibers havingupstream portions of low tensile strengths. As depicted therein, sidesupports 11 support a multiple curved surface having a descending firstconcave region 13 and an ascending second concave region 14 which flexesinto a convex region IS. A rotating member 16 for take-up of a fiber ispositioned adjacent tensioning device 10.

The first concave region is positioned to intercept a fiber 17 extrudedfrom crucible 18 which fiber for purposes of illustration is formed viaa film stabilizing technique. Thus, the point of interception 19 may befurther defined as lying between points D, and D along the fiber path.The operating effect of device is to provide the tensional force to thefiber yet ism late the same force from the liquid upstream portion. Dueto its centrifugal force, fiber 17 is constrained to slide across theconcave regions 13, 14 of the multiple curved surface. The interactionbetween fiber 17 and the surface causes a frictional force, the force F,in equation (4), to continually build up, and with contributions fromaerodynamic drag, gravity, and friction due to the weight of the fiber,the tension reaches a value of about pAv". Atthis point, fiber 17 hassufiicient tension to form an unsupported loop 20. Convex region ispositioned above the point at which the fiber forms the loop thus thefiber moves both up and over convex region 15 enroute to rotating member16.

The tension build up in a fiber due solely to the frictional interactionof the fiber moving against a concave surface because of its centrifugalforce may be represented by dT pAu T where [L is the coefficient offriction between the fiber and concave surface;

r is the radius of curvature of the concave surface;

x is the distance along the surface, cm;

Tis the tensional force on the fiber, dynes;

p is the fiber density, gms/cm;

A is the fiber cross-sectional area, cm

v is the fiber velocity, cm/sec.

As can be seen from equation (5), the value of Tapproaches pAv as alimit. Thus, for values of T near Av the concave surface is ineffectiveto cause significant increases in tension. It may-further be shown thataerodynamic drag and gravity are the most significant factors inincreasing the tension in the fiber during the last 10 percent or so oftravel along concave surface where T is near pAv Because as explainedbelow, it is highly desirable to attain an unsupported loop in thefiber, it is preferable in many instances to enhance the effect ofgravity by utilizing a concave surface with an ascent or up-slopeportion (such as region 14 in FIG. 2) immediately following the descentportion. Relationship (5) also illustrates the importance of the radiusof curvature of the curved surface. The smaller the radius, i.e., themore pronounced the curvature, the shorter is the length of surfacenecessary to approach pAv all other factors being constant.

As stated before, a convex surface is ineffective to increase thetensional force in a fiber unless the fiber already possesses a forcegreater than pAv The force of gravity acting upon the fiber as it movesup to the convex surface increases the tensional force to a valueslightly greater than pAv The convex surface then via frictionalinteraction with the fiber further increases the tensional force to avalue preselected for good packaging about a bobbin. Although thestiffness of the wire does contribute to tensioning, the effect thereofis small and is neglected for purposes of this disclosure.

As can now be appreciated from the preceding discussion, the optimumshape of the slide is determined by the relative importance of l.tension build up due to the moving fiber being constrained against theconcave portion by centrifugal force,

2. tension build up due to aerodynamic drag;

3. tension build up due to friction caused by the force of gravity,

4. tension build up due to movement with or against gravity. Thus, itfollows that the shape depends upon the weight of the fiber, coefficientof friction, and fiber velocity.

The presence of unsupported loop not only indicates that a tensionalforce of about pAv has been induced into fiber 17, but also indicatesthat self-regulation is present. When the take-up speed of fiber 17and/or extrusion velocity is changed, the unsupported loop generallyforms at a different position along the slide. This is because thetensional force relationships are changed.

The extent of self-regulation is limited primarily by the possibledifferential between the extrusion speed and take-up speed, the take-upspeed being a value as large or larger than the extrusion speed. It hasfurther been found that the take-up speed can exceed the extrusion speedby as much as 20 percent, i.e., V 1.20 V, where V is the extrusion speedand V is the take-up speed. The efi'ect of a take-up velocity greaterthan an extrusion velocity is to attenuate the molten stream to asmaller diameter which under many circumstances is highly desirable.

The angle of interception a, i.e., the angle between concave region 13and fiber 17, as seen in FIG. 2 should have a value such that the forcesgenerated in striking concave surface region 13 are absorbed in bendingthe fiber to follow the slide contour rather than creating tensionalforces. It may be shown that the force on the fiber at impact is pAv cosa where T is the force in the wire before impact in dynes, and T is theforce in the wire after impact in dynes. At very small values for a, theexact point of contact with the slide would vary considerably for smalllateral displacements. At large values of a, the bending moment of thefiber causes the tensional force in the falling fiber to exceed themaximum tensional force F, (max) of equation (1). Thus, it has beenfound that the intercept angle a should have a value with optimumresults occurring between 20 and 45.

It should now be apparent to those skilled in the art that thefrictional characteristics between the fiber and surface may be variedas desired, for example,,through the use of magnets (for fibers ofmagnetic materials), oil, or air currents. In FIG. 2, magnets 21 (shownin dotted outline) are appropriately positioned to enhance frictionbetween the concave surface and, for example, steel fibers.

In certain instances when either or both the coefficient of friction ofthe multiple curved surface of FIG. 2 and the fiber are small and/or thetake-up speed is large, the distance from the interception point to theunsupported loop may become large enough to require an extremely longconcave region. When space is limited, long friction surface lengthsbecome highly inconvenient or impractical. Reference is made to theembodiment illustrated in FIG. 3 which is specifically structured tominimize the amount of space required to tension a fiber. Tensioningdevice 30 comprises a plurality of concave friction surfaces 31 arrangedso as to have the bottom portion or base of each surface positionedadjacent to and in a noncontacting relationship with the top portion ofa concave surface therebeneath. The base of the bottom surface 31 ispositioned. adjacent to a convex surface 33. A rotating take-up meanssuch as bobbin 34 is positioned adjacent surface 33.

In operation, the upper surface 31 functions to intercept fiber 35 whichin turn remains in contact with concave surfaces 31 until it reachesbottom surface 31. At a portion along bottom surface 31, the tensionalforce induced into fiber 35 reaches the magnitude necessary for theformation of an unsupported loop. As in the embodiment of FIG. 2, theadditional tensional force is provided by the frictional interaction ofthe moving fiber with the convex surface 33. Convex surface 33 is spacedabove the base of bottom surface 31. The properly tensioned fiber isthen wound around rotation bobbin 34 Still another embodiment isillustrated by FIG. 4 in which the fiber 40 is constrained to pass overtwo concave surfaces 41 and 42, one positioned over the other. Thetensional force in the fiber reaches about a value pAv at a pointsomewhere along surface 42. As in the embodiments of FIGS. 2 and 3 thetension force may further be increased by causing the fiber to moveupward against the force of gravity and/or passing it over a convexsurface, 43 to a rotating take-up bobbin 44.

Surface depicted in FIG. illustrates that, while fiber 50 must move in aconcave path, the surface itself need not be completely concave but onlypresent the general appearance of concavity. Fiber 50 touches only thepeaks 52 of surface 51 as it moves thereacross. As before, a convexsurface 53 may be used to increase the tension beyond Av prior totake-up on bobbin 54.

FIG. 6 illustrates a limiting case of the embodiment of FIG. 3 throughwhich fiber 60 is constrained by gravity and the position of plates 6],to move against plates 61. That is, the centrifugal forces experiencedby fiber 60 is always in a direction toward plates 61. The tensionalforce is thereby increased to a value of about pAv. Reaching thistensional value, fiber 60 forms an unsupported loop. Additional platescannot further increase the tensional forces, thus making it necessaryto increase the tensional force through other means such as convexsurface 62, for example, to facilitate take-up about rotating bobbin 63.

To better illustrate the present invention, reference is now made to thefollowing example.

EXAMPLE A tensioning device similar to that depicted in FIG. 2 wasutilized to collect and take-up continuous aluminum fiber. The multiplecurved surface was fabricated from 0.037 inch thick sheet steel againstwhich an aluminum wire has a coefficient of friction of approximately0.7. The radius of curvature of the surface varied from about 7 feet inthe descending portion to about 2 feet in the ascending portion adjacentto the convex surface. The radius of curvature of the convex surface wasapproximately 10 inches. The diameter of the extrusion orifice was about7.2 mils. The density of the molten aluminum utilized for fabricatingthe metal fiber was approximately 2.3 gms/cm.

In operation, the initial extrusion velocity of the aluminum stream atthe orifice was 690 ft/minute. Oxygen present in the atmosphere underapproximately room temperature and pressure conditions was utilized tostabilize the molten stream. When spinning a metal wire of this diametercomprised essentially of aluminum at this velocity, the concentration ofoxygen in air is sufficient to ensure that D,, lies below D,,. This wasdetermined by moving an intercepting surface along the stream untilcontinuous fibers were formed. Using the descending portion of theconcave surface, the stream was intercepted slightly below the D pointat about 10 feet below the orifice at an angle of about 30. As the fibermoved down the surface, the lead end was manually picked up and passedover the convex surface region into a self-feeding winder mechanism. Thelatter was essentially a rotating bobbin and a transverse feed device toensure proper positioning on the bobbin.

The unsupported loop was observed to form at about 6.5 feet from theinterception point along the ascending portion. Attenuation of the fiberwas mainly attributed to the net tension force formed on the fiber sincethe fiber velocity at interception was essentially the same as thetake-up speed, i.e., about 700 ft/minute.

Table I indicates that with increasing take-up speeds thecross-sectional diameter of the fiber decreases.

TABLE I Take-Up Speed, ft/min. Fiber Diameter, mils 700 (350 cm/sec) 7.1(.0l8 cm) 725 (362 cm/sec) 6.96 (.0l76 cm) 750 (375 cm/sec) 6.8 (.0173cm) 775 (387 cm/sec) 6.7 (.0l7 cm) 800 (400 cm/sec) 6.6 (.Ol67 cm) 825(4l2 cm/sec) 6.5 (.0164 cm) The smaller diameter at a take-up speed of700 ft/min. reflects the attenuation due primarily to the weight of thefiber extending from the orifice to the interception point, i.e.,gravity attenuation. The important feature illustrated by Table I isthat small but significant changes in take-up speeds and/or extrusionvelocities are not detrimental to continuous fiber formation andtake-up.

SUMMARY Although the prior art has employed chutes and the like tocollect various materials, no one heretofore has been successful in thein-line take-up of extruded fibers with fragile upstream portions.Applicants, however, recognizing that prior attempts to take-up suchfibers have failed because of tensioninduced breakage in the upstreamportions, have not only isolated the needed take-up tension from theupstream portion but have provided a self-regulating take-up apparatusand method which is independent of variations in the extrusion andtake-up speed of the fiber.

From the foregoing discussion, it is now apparent that the presentinvention involves the concept of positioning concave and/or fiatsurfaces so as to constrain a moving fiber to move against the surfaces.The total length of the surface or surfaces is sufficient to cause africtional interaction between the fiber and surface or surfaces whichcreates a tensional force to develop in the fiber which increases invalue toward a limiting value of .411". Other important contributions toincreasing the tension in the fiber to the desired value of pAu aredrag, gravity, and fiber weight. At a value of about pAv the fiber formsan unsupported loop. Because the take-up of a fiber requires a value ofsomewhat more than pAv", it is necessary to further increase the tensionin the fiber. This may be accomplished in various ways, the use of whichordinarily depends upon the magnitude of tensional force increaseneeded. One way is to pass the fiber over a convex surface positioned ata selected distance above the point at which the unsupported loop forms.The force of gravity increases the tensional force to a value whichconstrains the fiber to move against the convex surface. The frictionalinteraction between the fiber and convex surface further increases thetensional force.

Although it is preferred to use a second tensioning means, such as aconvex surface prior to wind-up, under special conditions the take-upmeans may be used as a second tensioning means by winding the fiberdirectly after the unsupported loop is formed, utilizing only the effectof gravity to increase the tension above pAv It should be apparent thatwhile applicants have described the present invention in connection withvertical extruding assemblies it is also adaptable for use withnon-vertical spinning assemblies as well.

It is now apparent that the various embodiments of the present inventionattain the objects and advantages as described. Thus, the inventionhaving been set forth with respect to certain embodiments and examplesthereof, those skilled in the art will become readily aware of the manymodifications and changes obtainable in light of the descriptive mattercontained herein. Accordingly, the appended claims are intended to coverall such modifications and changes as fall within the true spirit andscope of the present invention.

What is claimed is:

1. Apparatus for taking up a continuous fiber moving at a velocity vcomprising a. a source for a continuous fiber having a low tensilestrength in the upstream region.

b. first tensioning means including at least one surface situatedbeneath said fiber source for intercepting said fiber so as to constrainsaid fiber to move along said surface until a tensional force of aboutpAv is induced into said fiber, p and A being, respectively, the densityand crosssectional area of said fiber, at which position along saidsurface the fiber forms an unsupported loop, and

c. means for increasing the tensional force on the fiber to a valueabout pAv including at least one convex surface engaging saidunsupported loop and means for taking up said fiber under a tensiongreater than pAv 2. The apparatus of claim 1 in which the take-up meansis a rotating bobbin spaced above the point at which the tension in thefiber is about pAv a vertical distance sufficient to increase thetension in the fiber to a value above pAu prior to winding about thebobbin.

3. The apparatus of claim 1 in which the first tensioning means isessentially a concave surface.

4. The apparatus of claim 3 in which the concave surface intercepts thefiber at an angle of between to 90.

5. The apparatus of claim 4 in which the intercept angle is between 20to 45.

6. The apparatus of claim 1 in which the first tensioning means includesa plurality of surfaces positioned to constrain a moving fiber to a pathwhich causes the fiber to have centrifugal forces in directions towardthe surfaces.

7. The apparatus of claim 6 in which the surfaces are essentially Hat.

8. The apparatus of claim 7 in which the surfaces are essentiallyconcave.

9. The apparatus of claim 8 in which the concave surfaces are arrangedin descending order with the bottom portion of each surface positionedadjacent to a top portion of the concave surface therebeneath so as topresent a substantially downward path to the fiber moving across thesurfaces.

10. A tensioning device for use in the wind-up of a continuous fiberhaving an upstream portion of low tensile strength comprising a. asupport;

b. a multiple curved upper surface supported by said support, saidmultiple curved surface having 1. a concave region with a descendingfirst portion thereof for intercepting a downwardly moving continuousfiber at a preselected point along said surface and an ascending secondportion wherein the length of said concave region from said preselectedpoint, the coefficient of friction between said surface and the fibermoving thereacross, and the height of the ascending portion aresufficient to cause the fiber to form an unsupported loop, and

2. a convex region above said ascending second portion for impartingfrictionally-induced tensional force to a fiber moving thereacross.

11. A method for taking-up an extruded continuous fiber having a lowtensile strength in the upstream region comprismg a. constraining thefiber to move in a curved path with at least one surface member so as toinduce a tensional force sufficient to cause the fiber to form anunsupported loop and b. winding the fiber about a rotating member.

12. The method of claim 11 in which the rotating member is spaced abovethe point at which the unsupported loop is formed a distance sufficientto cause an increase in the tensional force of the fiber.

13. The method of claim 11 in which the unsupported loop is terminatedby passing the fiber over a convex surface positioned above the point atwhich the unsupported loop is formed.

14. A process for taking-up an extruded continuous fiber having a lowtensile strength in the upstream region thereof comprising extruding thefiber against a concave surface at a velocity V passing the fiber acrossthe concave surface until the fiber forms an unsupported loop,terminating the unsup-' ported loop on a convex surface and passing thefiber across the convex surface, and winding the fiber about a rotatingmember. 4

15. The process of claim 14 wherein the angle between the concavesurface and the extruded fiber is between about 0to 16. The process ofclaim 14 wherein the angle is between about 20 to 45.

17. The process of claim 14 in which the fiber is extruded as a moltenstream and subsequently stabilized pending solidification wherein thefiber is wound around the rotating members at a velocity V lying betweenV and V

1. Apparatus for taking up a continuous fiber moving at a velocityupsilon comprising a. a source for a continuous fiber having a lowtensile strength in the upstream region. b. first tensioning meansincluding at least one surface situated beneath said fiber source forintercepting said fiber so as to constrain said fiber to move along saidsurface until a tensional force of about Rho A2 is induced into saidfiber, Rho and A being, respectively, the density and crosssectionalarea of said fiber, at which position along said surface the fiber formsan unsupported loop, and c. means for increasing the tensional force onthe fiber to a value about Rho A2 including at least one convex surfaceengaging said unsupported loop and means for taking up said fiber undera tension greater than Rho A2.
 2. The apparatus of claim 1 in which thetake-up means is a rotating bobbin spaced above the point at which thetension in the fiber is about Rho A2 a vertical distance sufficient toincrease the tension in the fiber to a value above Rho A2 prior towinding about the bobbin.
 2. a convex region above said ascending secondportion for imparting frictionally-induced tensional force to a fibermoving thereacross.
 3. The apparatus of claim 1 in which the firsttensioning means is essentially a concave surface.
 4. The apparatus ofclaim 3 in which the concave surface intercepts the fiber at an angle ofbetween 0* to 90*.
 5. The apparatus of claim 4 in which the interceptangle is between 20* to 45*.
 6. The apparatus of claim 1 in which thefirst tensioning means includes a plurality of surfaces positioned toconstrain a moving fiber to a path which causes the fiber to havecentrifugal forces in directions toward the surfaces.
 7. The apparatusof claim 6 in which the surfaces are essentially flat.
 8. The apparatusof claim 7 in which the surfaces are essentially concave.
 9. Theapparatus of claim 8 in which the concave surfaces are arranged indescending order with the bottom portion of each surface positionedadjacent to a top portion of the concave surface therebeneath so as topresent a substantially downward path to the fiber moving across thesurfaces.
 10. A tensioning device for use in the wind-up of a continuousfiber having an upstream portion of low tensile strength comprising a. asupport; b. a multiple curved upper surface supported by said support,said multiple curved surface having
 11. A method for taking-up anextruded continuous fiber having a low tensile strength in the upstreamregion comprising a. constraining the fiber to move in a curved pathwith at least one surface member so as to induce a tensional forcesufficient to cause the fiber to form an unsupported loop and b. windingthe fiber about a rotating member.
 12. The method of claim 11 in whichthe rotating member is spaced above the point at which the unsupportedloop is formed a distance sufficient to cause an increase in thetensional force of the fiber.
 13. The method of claim 11 in which theunsupported loop is terminated by passing the fiber over a convexsurface positioned above the point at which the unsupported loop isformed.
 14. A process for taking-up an extruded continuous fiber havinga low tensile strength in the upstream region thereof comprisingextruding the fiber against a concave surface at a velocity V1, passingthe fiber across the concave surface until the fiber forms anunsupported loop, terminating the unsupported loop on a convex surfaceand passing the fiber across the convex surface, and winding the fiberabout a rotating member.
 15. The process of claim 14 wherein the anglebetween the concave surface and the extruded fiber is between about 0*to90*.
 16. The process of claim 14 wherein the angle is between about 20*to 45*.
 17. The process of claim 14 in which the fiber is extruded as amolten stream and subsequently stabilized pending solidification whereinthe fiber is wound around the rotating members at a velocity V2 lyingbetween V1 and 1.20V1.